The present inventions are related to systems and methods for compensating signals received from a magnetoresistive head, and more particularly to systems and methods for determining the applicable compensation for distortion introduced by a magnetoresistive head.
Some storage devices rely on magnetoresistive read heads to sense information previously written to a magnetic storage medium. Such heads typically exhibit some level of non-linear distortion that causes significant degradation in the performance of a data detection circuit, and in some cases disqualification of magnetoresistive heads where the degradation becomes too large. To avoid this degradation, various approaches have been developed to compensate for the non-linear distortion. Existing approaches are, however, costly in terms of time required to calculate distortion compensation or due to high latency the distortion compensation calculation typically must be performed only at start-up and in reasonably static conditions.
Some existing approaches for distortion compensation rely on post detection data output to determine compensation factors. Turning to FIG. 1, an example of a post detection distortion compensation system 100 is shown. Post detection distortion compensation system 100 includes a magnetoresistive read head 130 that senses magnetic information and provides a corresponding electrical signal to an analog signal conditioning circuit 135. A condition analog signal is provided to a variable gain amplifier 140. The output from variable gain amplifier 140 is provided to an analog squaring function 145 (i.e., a second order function). The result of the second order function is multiplied by a distortion compensation factor 152 using a multiplication circuit 150, and the product of the multiplication is subtracted from the output of variable gain amplifier 140 using a summation element 155.
The result from summation element 155 is provided to a continuous time filter 105, and the filtered output is provided to an analog to digital converter 110. The corresponding series of digital samples derived from analog to digital converter 110 are provided to a digital detection circuit 115 that includes a data detector. Digital detection circuit 115 provides a data output 125, and also provides an error information to an adaptive distortion compensation calculation circuit 120. Using the error information, adaptive distortion compensation calculation circuit 120 provides distortion compensation factor 152. Of note, the distortion compensation factor is not available until a considerable time after the data from which it was derived is received at magnetoresistive head 130. This latency renders such an approach ineffective for real-time computation of distortion compensation factors.
Searching over a grid is the most commonly used approach to determine the amount of asymmetry, and thereby needed compensation. Such an approach is quite time-consuming and also ineffective for real-time computation of distortion compensation factors. Turning to FIG. 2, a flow diagram 200 depicts a method for searching over a grid for distortion compensation factors. Following flow diagram 200, an initial compensation parameter value is selected and set (block 205). A detection process is performed (block 210), and a resulting error rate is determined and stored (block 215). It is determined if another parameter value is to be tested (block 220). Where another parameter is to be tested (block 220), the next parameter value is selected and set (block 207) and the processes of blocks 210-220 re repeated for the next parameter value. Where no parameter values remain to be tested (block 220), the stored error rates are compared (block 225) and the parameter corresponding to the lowest error rate is selected (block 230) as the distortion compensation parameter.
The approach of FIG. 2, similar to that discussed above in relation to FIG. 1, relies on back-end information (e.g., data-decisions) for estimating the distortion compensation. This becomes a problem when there is substantial latency between front-end and back-end. Further, the approaches of both FIG. 1 and FIG. 2 require gain normalization in the signal prior to determining distortion compensation parameters. This limits flexibility, and it becomes a serious problem in system where the front-end and back-end are substantially decoupled. Finally, in AC-coupled channels, MRA compensation reintroduces DC-content in the signal and worsens the baseline wander caused by the AC-coupler.
Hence, for at least the aforementioned reasons, there exists a need in the art for advanced systems and methods for determining fly-height.